During a production process of metal can bodies such as seamless can bodies produced by drawing with ironing, for example (hereinafter simply referred to as “can body”), pinholes in the form of holes or cracks may be created in a body part or the like of the can body. Commonly, presence or absence of such pinholes is determined by using a pinhole inspection apparatus in an inspection process of can bodies.
The pinhole inspection apparatus shown in Patent Document 1 is configured, as shown in FIG. 10, with an inspection turret that holds can bodies to be inspected on the right side, and an apparatus for inspecting the interior of can bodies on the left side. A piston-like sealing member is provided with a sealing ring plate fixedly attached thereto for sealing a sliding contact surface with a sliding ring plate by making sliding contact therewith. The piston-like sealing member is mounted on a distal end face of a first frame on the side facing the sliding ring plate for making the sliding contact. During pinhole inspection, the piston-like sealing member is pressed against the sliding ring plate rotating at a high speed with air pressure, to enhance the sealing effect at the sliding contact surface, and to prevent entrance of ambient light, which is formed of outside light and light from a light source, through the sliding contact surface to a photodetector side.
Since the sealing ring plate is kept in pressure contact with the rapidly rotating sliding ring plate during pinhole inspection as noted above, ambient light normally does not reach the photodetector through the sliding contact surface between the sealing ring plate and the sliding ring plate. However, there was a possibility that ambient light may reach the photodetector through a gap that may be created instantaneously between the sealing ring plate and the sliding ring plate due to various factors such as some form of external force applied thereto, or surface conditions of these plates, and good products may be erroneously determined as defective.
The diameter of pinholes to be inspected by a conventional pinhole inspection apparatus for can bodies is about 20 μm, which is determined by the performance of the photodetector. Pinholes with an even smaller diameter may not pass an enough amount of light into the can body to be detectable. A possible approach would be to secure a sufficient amount of light for enabling detection by the photodetector by using a light source with higher brightness than conventional light sources. However, the conventional apparatus configuration does not have sufficient countermeasures against ambient light, and the increase in the light amount could lead to an increased erroneous detection rate.
Patent Document 2 describes shielding light from a light source with a peripheral portion and an inner edge portion made of aluminum coated with black silicone resin. Patent Document 3 describes a light shield part (cover) that can block light from a light source and prevent light from penetrating between a movable plate and a rotating disc. Neither of these configurations has not fully resolved the problems described above, as ambient light may still pass around the peripheral portion and inner edge portion, or the light shield part (cover), and reach the photodetector side from between the movable plate and rotating disc.